Excimer lasers are typically configured with two opposing mirrors that define a resonant cavity, and a gain medium placed therebetween. The gain medium is a gas mixture containing halogen and rare gases that is excited using electrodes to generate an intracavity laser beam that oscillates between the mirrors. Typical excimer laser systems cyclically activate the electrodes to generate a pulsed intracavity laser beam. One of the mirrors is partially transmissive to produce an output laser beam.
Excimer lasers often use a planar diffraction grating as a wavelength selector to force the laser to oscillate in a narrow linewidth. The grating reflects light of different wavelengths at different angles. The grating is aligned so that only the desired linewidth of the light reflecting therefrom is redirected back along the resonant cavity axis. Other non-desired spectral components of the laser light are reflected back to the gain medium at some angle to the optical axis of the resonator, and therefore suffer increased losses which prevent their oscillation in the resonant cavity.
In order to increase the resolution of the grating, a beam expander can be used to expand the beam and reduce its divergence by the same proportion before the beam reaches the grating. An expanded beam with less divergence gives the grating better resolution, which results in a narrower linewidth oscillating in the resonant cavity.
Even with beam expanders, there tends to be curvature in the wavefront of the light passing to the grating, which broadens the spectral linewidth of the laser since different portions of the curved wavefront strike the planar grating at slightly different angles. Therefore, spectral components of different wavelengths can be retroreflected back onto themselves and oscillate in the resonant cavity. The curved wavefront is caused by imperfections of optical components and intracavity windows, diffraction from intracavity apertures, and by a nonuniform gain profile and refractive index of the gain medium. The amount of wavefront curvature can vary during laser operation as components age and as operating conditions and optical alignment change. For example, it is well known in the art that optical elements of high average power lasers are generally subject to so called "thermal lensing" effects which is essentially power dependent through the temperature dependence of refractive index and linear expansion and distortions. Therefore, there has been a practical limit on how narrow and stable the linewidth of such a laser system can be made.
One solution to this problem is discussed in U.S. Pat. No. 5,095,492, issued to Sandstrom on Mar. 10, 1992, where the grating is bent to match its surface curvature to that of the incoming wavefront. Thus, only a very narrow band of wavelengths is reflected back along the resonant cavity axis for re-amplification. The curvature of the grating can be adjusted by changing controlled forces applied to the grating at three spaced points, which bend the grating into the desired grating curvature.
Bending the grating to match its shape to that of the incoming wavefront, however, has several disadvantages. First, applying variable forces to the grating can cause some uncontrollable distortions to the grating surface, thus limiting the precision of curvature compensation by the grating. Secondly, the grating is subject to mechanical strain which may cause the grating surface curvature to be unstable over time. Lastly, the amount of possible wavefront curvature compensation is limited. Too large a compensation can permanently bend or break the grating. Further, since the compensation is being made after the beam is expanded by the beam expander, where the divergence is smaller, the compensation is less precise.
There is a need for a simple yet reliable wavefront curvature compensator that can compensate for large variations of wavefront curvature in the intracavity beam with precision and long term stability.